Re: [Qemu-devel] [QEMU 1/7] balloon: speed up inflating & deflating process

[Thomas Huth <thuth@redhat.com>]

On 13.06.2016 12:16, Liang Li wrote:
> The implementation of the current virtio-balloon is not very efficient,
> Bellow is test result of time spends on inflating the balloon to 3GB of
> a 4GB idle guest:
> 
> a. allocating pages (6.5%, 103ms)
> b. sending PFNs to host (68.3%, 787ms)
> c. address translation (6.1%, 96ms)
> d. madvise (19%, 300ms)
> 
> It takes about 1577ms for the whole inflating process to complete. The
> test shows that the bottle neck is the stage b and stage d.
> 
> If using a bitmap to send the page info instead of the PFNs, we can
> reduce the overhead spends on stage b quite a lot. Furthermore, it's
> possible to do the address translation and do the madvise with a bulk
> of pages, instead of the current page per page way, so the overhead of
> stage c and stage d can also be reduced a lot.
> 
> This patch is the QEMU side implementation which is intended to speed
> up the inflating & deflating process by adding a new feature to the
> virtio-balloon device. And now, inflating the balloon to 3GB of a 4GB
> idle guest only takes 210ms, it's about 8 times as fast as before.
> 
> TODO: optimize stage a by allocating/freeing a chunk of pages instead
> of a single page at a time.
> 
> Signed-off-by: Liang Li <liang.z.li@intel.com>
> ---
>  hw/virtio/virtio-balloon.c                      | 159 ++++++++++++++++++++----
>  include/standard-headers/linux/virtio_balloon.h |   1 +
>  2 files changed, 139 insertions(+), 21 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 8c15e09..8cf74c2 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -47,6 +47,76 @@ static void balloon_page(void *addr, int deflate)
>  #endif
>  }
>  
> +static void do_balloon_bulk_pages(ram_addr_t base_pfn, int page_shift,
> +                                  unsigned long len, bool deflate)
> +{
> +    ram_addr_t size, processed, chunk, base;
> +    void *addr;
> +    MemoryRegionSection section = {.mr = NULL};
> +
> +    size = (len << page_shift);
> +    base = (base_pfn << page_shift);
> +
> +    for (processed = 0; processed < size; processed += chunk) {
> +        chunk = size - processed;
> +        while (chunk >= TARGET_PAGE_SIZE) {
> +            section = memory_region_find(get_system_memory(),
> +                                         base + processed, chunk);
> +            if (!section.mr) {
> +                chunk = QEMU_ALIGN_DOWN(chunk / 2, TARGET_PAGE_SIZE);
> +            } else {
> +                break;
> +            }
> +        }
> +
> +        if (section.mr &&
> +            (int128_nz(section.size) && memory_region_is_ram(section.mr))) {
> +            addr = section.offset_within_region +
> +                   memory_region_get_ram_ptr(section.mr);
> +            qemu_madvise(addr, chunk,
> +                         deflate ? QEMU_MADV_WILLNEED : QEMU_MADV_DONTNEED);
> +        } else {
> +            fprintf(stderr, "can't find the chunk, skip\n");

Please try to avoid new fprintf(stderr, ...) in the QEMU sources.
Use error_report(...) or in this case maybe rather
qemu_log_mask(LOG_GUEST_ERROR, ...) instead, and try to use a more
reasonable error message (e.g. that it is clear that the error happened
in the balloon code).

> +            chunk = TARGET_PAGE_SIZE;
> +        }
> +    }
> +}
> +
> +static void balloon_bulk_pages(ram_addr_t base_pfn, unsigned long *bitmap,
> +                               unsigned long len, int page_shift, bool deflate)
> +{
> +#if defined(__linux__)

Why do you need this #if here?

> +    unsigned long end  = len * 8;
> +    unsigned long current = 0;
> +
> +    if (!qemu_balloon_is_inhibited() && (!kvm_enabled() ||
> +                                         kvm_has_sync_mmu())) {
> +        while (current < end) {
> +            unsigned long one = find_next_bit(bitmap, end, current);
> +
> +            if (one < end) {
> +                unsigned long zero = find_next_zero_bit(bitmap, end, one + 1);
> +                unsigned long page_length;
> +
> +                if (zero >= end) {
> +                    page_length = end - one;
> +                } else {
> +                    page_length = zero - one;
> +                }
> +
> +                if (page_length) {
> +                    do_balloon_bulk_pages(base_pfn + one, page_shift,
> +                                          page_length, deflate);
> +                }
> +                current = one + page_length;
> +            } else {
> +                current = one;
> +            }
> +        }
> +    }
> +#endif
> +}

 Thomas